Don't rely on responseXML, also use responseText

On servers that report an incorrect content type for XML
documents, responseXML will be empty. In these cases we need to
use responseText instead. This change also prevents readNode
calls with empty data.
Use goog.isString() instad of typeof check
Merge pull request #713 from ahocevar/kml-responsetext

Do not rely on responseXML, also use responseText. r=@bartvde